honor get_templates_path() first

honor get_templates_path() first

Fixed regression caused by the ``pathlib`` refactoring that removed the use
of :meth:`.Config.get_template_directory` as the canonical source of
templates; the method is still present however it no longer would be
consulted for a custom config subclass, as was the case with flask-migrate.

Fixed regression caused by the ``pathlib`` refactoring where the "missing
template" error message failed to render the name of the template that
could not be found.

Fixes: #1659
Fixes: #1660
Change-Id: I1244f19c9f288eb52c8e9ffcc3835824d6b74bb6